I think that in extras.rsrc all of the resources are in the data fork and not the resource fork so that's probably where your confusion lies. You can just read the data fork into an NSData object. On the other hand, if you need more flexibility, which is likely since NSData objects aren't as useful as they could be, then just use some file manager functions. If you have the developer tools installed, then you can find the documentation at
file:///Developer/Documentation/Carbon/Files/FileManager/File_Manager/File_Manager.html
Hint: You can still use the standard unix style of paths when you use the file manager functions. Just use the FSPathMakeRef function to convert it into an FSRef for use with the file manager.